Android

I know that there are similar question like this, but none of the solutions work for me.
I want to create a folder in my root directory like whatsapp but i did not get it to work.
I was able to create a folder in my package folder but same method did not work for root directory

    //works for creating folder in my package directory
    File pk = new File(this.getExternalFilesDir("/myweb").getAbsolutePath());
    
    pk.mkdir();
    
    
    //this did not work for creating folder in root directory like whatsapp
    File root = new File(Environment.getExternalStorageDirectory() + "/myweb");
    
    root.mkdir();

How can i solve this problem

<Frame BackgroundColor="Black"
                  CornerRadius="10"
                  HasShadow="False"
                  Opacity="0.5"
                  Margin="30">
       <FlexLayout Direction="Column"
               JustifyContent="SpaceEvenly"
               Padding="5">

           <StackLayout>
               <Label Text="قد شما چند سانتیمتر است؟"
                  Style="{StaticResource TitleStyle}"/>
               <Label Text="{Binding Source={x:Reference HeightSlider},
               Path=Value,
               StringFormat='{0:F0} cm'}"
                  Style="{StaticResource ValueStyle}"/>
               <Slider x:Name="HeightSlider"
                   Maximum="220"
                   Minimum="40"
                   Value="{Binding Height}"/>
           </StackLayout>
           <StackLayout>
               <Label Text="وزن شما چند کیلوگرم است؟"
                  Style="{StaticResource TitleStyle}"/>
               <Label Text="{Binding Source={x:Reference WeightSlider},
               Path=Value,
               StringFormat='{0:F0} kg'}"
                  Style="{StaticResource ValueStyle}"/>
               <Slider x:Name="WeightSlider"
                   Maximum="150"
                   Minimum="5"
                   Value="{Binding Weight}"/>
           </StackLayout>
           <StackLayout>
               <Label Text="شاخص توده بدنی شما:"
                  Style="{StaticResource LabelStyle}"/>
               <Label Text="{Binding BMI}"
                  Style="{StaticResource LabelStyle}"
                  FontSize="48"/>
               <Label Text="{Binding Classification}"
                  Style="{StaticResource LabelStyle}"
                  FontSize="20"/>
           </StackLayout>
       </FlexLayout>
   </Frame>